Refactoring: Remove unused function TrackPossibleClickBurst

There is no caller of TrackPossibleClickBurst, and there is no
implementation of it.

Change-Id: I9899ced69e6902f88e9fdeb85b0007107bf50bbb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2134026
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#755812}

components/page_load_metrics/browser/observers/core_page_load_metrics_observer.h

@@ -174,7 +174,6 @@ class CorePageLoadMetricsObserver
                                  int behavior_flags) override;
 
  private:
-  void TrackPossibleClickBurst(const blink::WebInputEvent& event);
   void RecordTimingHistograms(
       const page_load_metrics::mojom::PageLoadTiming& main_frame_timing);
   void RecordByteAndResourceHistograms(